Recently, digital cameras and video cameras using CCD and CMOS type solid-state image sensors have generally been in widespread use. Such a digital camera or video camera is equipped with an auto-focus (to be abbreviated as AF hereinafter) function to automatically adjust the focus position of an imaging lens. Compact cameras and video cameras perform contrast system AF to adjust a focus state by evaluating the contrast of an image signal.
In a digital single-lens reflex camera, when performing still image capturing by using an optical finder, a dedicated phase difference focus detecting device performs AF by splitting an image light beam into light beams for the optical finder and a focus detection means using a quick return mirror. In electronic viewfinder image capturing or moving image capturing, the camera retracts the quick return mirror to guide an image light beam to only the solid-state image sensor, thereby performing contrast system AF like compact cameras and video cameras.
A dedicated phase difference focus detecting device, however, is disadvantageous in terms of space and cost. The contrast system is incapable of quick focus adjustment because it searches for a position where the contrast of an image signal is maximized while changing the focus position of the imaging lens.
In order to solve the above problems, there has been proposed a technique of providing a pupil dividing function for some light-receiving elements (pixels) of an image sensor by decentering the sensitivity region of a light-receiving portion relative to the optical axis of an on-chip microlens and using these pixels as focus detecting pixels. Arranging focus detecting pixels among image forming pixels at predetermined intervals can implement fast phase difference system AF even in electronic viewfinder image capturing or moving image capturing (see Japanese Patent Laid-Open No. 2000-156823).
In addition, there has been proposed a technique of, when performing fast readout operation by adding a plurality of pixel signals in electronic viewfinder image capturing or moving image capturing, adding and reading out pixel signals for the respective types so as not to mix signal outputs from the image forming pixels and the focus detecting pixels. This makes it possible to perform focus detection while suppressing a deterioration in the image quality of a displayed image (see Japanese Patent Laid-Open No. 2009-86424).
The following technique has also been proposed for an addition readout mode of reading out pixels from an image sensor while adding them to prevent a deterioration in image quality caused by moire fringes in electronic viewfinder image capturing or moving image capturing. That is, when targets for addition include focus detecting pixels, readout operation is performed so as not to mix signals from image forming pixels and signals from focus detecting pixels (see Japanese Patent Laid-Open No. 2010-20055).
Conventionally, however, when the electronic viewfinder mode or moving image capturing mode is set in an image capture apparatus designed to perform phase difference AF using an image sensor, the apparatus cannot perform focus detection while suppressing a deterioration in image quality.
For example, according to Japanese Patent Laid-Open No. 2000-156823, since the portions where the focus detecting pixels are arranged correspond to lost portions in terms of image signal, image information is generated by interpolation using adjacent image forming pixel information. It is therefore impossible to prevent a deterioration in image quality caused by interpolation errors. In addition, in the electronic viewfinder mode or moving image capturing operation, the apparatus reads out signals from pixels while thinning out pixels, resulting in a deterioration in image quality caused by moire fringes.
In addition, the technique disclosed in Japanese Patent Laid-Open No. 2009-86424 performs pixel addition in the electronic viewfinder mode or moving image capturing operation to suppress the generation of moire fringes, thus suppressing a deterioration in image quality. Likewise, however, since the portions where the focus detecting pixels are arranged correspond to lost portions in terms of image signal, image information is generated by interpolation using adjacent image forming pixel information. It is therefore impossible to prevent a deterioration in image quality caused by interpolation errors. Furthermore, it is necessary to transfer signals to a line memory for each line when performing pixel addition in the vertical direction. This leads to delays caused by vertical transfer, and hence makes it difficult to cope with a high frame rate.
According to Japanese Patent Laid-Open No. 2010-20055, when pixels to be added in the horizontal direction include focus detecting pixels, the addition result obtained without adding signals from the focus detecting pixels can be independently output. However, there is no specific description about addition in the vertical direction.